LED Resistor Calculator

Find the correct resistor for any LED - resistance, standard E24 value, power dissipation, and 4-band color code. Results update instantly.

Last reviewed: April 2026
Built and maintained by · Last updated April 2026
V
mA
Calculated Resistance
-
Nearest E24 Standard
-
Actual Current (E24)
-
Power Dissipation
-
Resistor Color Code (4-Band)
Power exceeds 0.25W - use a 1/2W or 1W resistor rated for higher wattage.
Important Disclaimer: The information provided by this calculator is for educational purposes only. It isn't intended to replace professional advice. Always consult a qualified electrical engineer or a licensed electrician for professional advice and guidance.

About the LED Resistor Calculator

The Led Resistor Calculator is an essential tool for anyone involved in electrical engineering, electronics, or DIY projects. LED resistors are used to limit the current flowing through an LED, ensuring it operates at the correct voltage and avoiding damage. The calculator simplifies the process of determining the appropriate resistor value based on the LED's forward voltage, desired current, and the supply voltage.

Understanding Ohms Law

Ohms Law is a fundamental principle in electrical circuits, stating that the current (I) flowing through a conductor between two points is directly proportional to the voltage (V) across the two points and inversely proportional to the resistance (R) between them. Mathematically, Ohms Law is expressed as V = IR.

LED Characteristics

LEDs have a forward voltage (Vf) and a forward current (If). The forward voltage is the voltage drop across the LED when it's forward-biased and conducting current. The forward current is the current flowing through the LED at a given forward voltage.

Calculating the Resistor Value

The Led Resistor Calculator uses Ohms Law to determine the appropriate resistor value (R) needed to limit the current through an LED. The formula for calculating the resistor value is R = (Vsupply - Vf) / If.

Real-World Applications

Example 1: You have an LED with a forward voltage of 2V and a forward current of 20mA. The supply voltage is 5V. Using the Led Resistor Calculator, you can determine the resistor value as follows:

R = (5V - 2V) / 20mA = 1.5V / 0.02A = 75 ohms

Counterexample: If you attempt to use a resistor with a value too low, the LED will draw too much current and may burn out. Conversely, if you use a resistor with a value too high, the LED won't receive enough current and may not light up.

Common Mistakes

What happens if you use the wrong supply voltage? If you use a supply voltage that's too high, it can damage the LED or the resistor. On the other hand, if you use a supply voltage that's too low, the LED may not light up or may not reach its full brightness.

Examples

Example 1: You have an LED with a forward voltage of 3V and a forward current of 10mA. The supply voltage is 12V. Using the Led Resistor Calculator, you can determine the resistor value as follows:

R = (12V - 3V) / 10mA = 9V / 0.01A = 900 ohms

Example 2: You have an LED with a forward voltage of 1.8V and a forward current of 25mA. The supply voltage is 5V. Using the Led Resistor Calculator, you can determine the resistor value as follows:

R = (5V - 1.8V) / 25mA = 3.2V / 0.025A = 128 ohms

Frequently Asked Questions

A1: You can verify the resistor value by plugging it into the Led Resistor Calculator and comparing the result with the value you calculated manually. If the values match, your calculation is correct. However, always consult a professional for critical applications.
A2: Using the wrong resistor value can lead to various issues, including damage to the LED or the resistor, insufficient current flow, or the LED not lighting up. It's essential to use the correct resistor value to ensure proper functionality.
A3: To choose the right resistor value for an LED, you need to know the LED's forward voltage, forward current, and the supply voltage. Use the Led Resistor Calculator to determine the appropriate resistor value based on these parameters. Always ensure that the resistor value is within the manufacturer's specifications to avoid any issues.